1. Field of the Invention
The present invention relates to a thermal head that is used as a printing device of a facsimile machine, a video printer or the like and a thermal printer.
2. Description of the Related Art
Conventionally, a thermal head is used as a printing device of a facsimile machine, a video printer or the like.
Such a known conventional thermal head comprises a substrate 101, a plurality of heating resistors 103 and an electrode pattern 104 connected to the heating resistors 103, which are mounted on an upper surface of the substrate 101, and a protective film 106 covering the heating resistors 103 and the electrode pattern 104, as shown in FIG. 11, for example.
A platen roller disposed directly above the heating resistors 103 delivers a recording medium M, such as image receiving paper, onto the heating resistors 103 while pressing the recording medium against the thermal head. At the same time, the heating resistors 103 are selectively made to generate heat by Joule heating based on externally input image data. The recording medium M comes into sliding contact with the surface of the protective film on the heating resistors 103, and the heat generated by the heating resistors 103 is transferred to the recording medium M via the protective film 106, and a printed image is thereby formed on the recording medium M.
The protective film 106 described above is made of a Si—O—N-based inorganic material having a high abrasion resistance and serves to protect the heating resistors 103 and the electrode pattern 104 against wear due to sliding contact with the recording medium M and corrosion due to exposure to moisture or the like in the atmosphere.
In the thermal head described above, if the recording medium M repeatedly comes into sliding contact with the surface of the protective film on the heating resistors, some of the static electricity on the surface of the recording medium M is accumulated on the protective film 106, and discharge occurs between the protective film 106 and the heating resistors 103, causing dielectric breakdown of the protective film 106. Thus, there is a problem that the protective film 106 loses functionality.
In order to solve the problem described above, it has been proposed that a conductive film made of an electrical resistive material of TaSiO is disposed on the protective film 106 (see Japanese Patent Laid-Open No. 2000-177158).
However, the surface of the recording medium M is rougher than the conductive film, and therefore, the frictional resistance between the recording medium M and the conductive film is particularly high in an area directly above the heating resistors and in the vicinity of the area where the pressing force of the platen roller is best transferred. Because of the frictional resistance, there arises a problem that the recording medium M is partially scraped off to produce many paper residues, and the paper residues hinder stable delivery of the recording medium M.
The present invention has been devised to solve the problems described above, and an object of the present invention is to provide a high-performance thermal head that allows a protective film to adequately serve for a long time and a recording medium M to be delivered stably and a thermal printer.